This invention is concerned with an underdrain for use with a granular filtration system, and more particularly, the invention relates to such an underdrain fitted with air and water backwash systems, and to an improvement in the air scouring system and the structure of the underdrain unit itself.
U.S. Pat. No. 4,331,542, assigned to the assignee of the present invention, describes an underdrain system for use with a gravity-fed granular filtration system, and one which is fitted with water backflushing and air scouring. The present invention is directed at an improvement over the underdrain system and air scouring subsystem described in that patent, as well as over other similar systems. U.S. Pat. No. 4,331,542 is incorporated herein by reference.
As described and shown in U.S. Pat. No. 4,331,542, high rate gravity filters typically include a filter bed of particulate filter material, usually gravel of varying sizes. Liquid from the tank or vessel flows downwardly through the filter medium and exits the filtration tank through openings in an underdrain structure which supports the filter medium. The medium may include different sizes of gravel, the coarsest at the bottom, against the underdrain structure, and the gravel decreasing in size up to the finest gravel or sand located at the top of the filter bed. Some beds are all sand. Such a filter bed may be covered with finely granulated anthracite coal or other types of filter material.
Gravity filters of this type usually are cleaned by backwashing, which may include a backflow of water or a combination of air (or gas) and water passed through the filter bed in the opposite direction of normal filtration. Uniform distribution of the backwashing water and the accompanying air, if included, is a goal of most underdrain structures. This includes the system described in the above-referenced patent, as well as some of the underdrain systems of the following patents: U.S. Pat. Nos. 4,659,462, 4,707,257, 5,015,383, 5,019,259, 5,156,738, 5,160,614, 5,269,920, 5,332,497, 5,462,664, 5,489,388, 5,512,174, 5,639,384.
A shortcoming of the prior underdrain systems, including that of U.S. Pat. No. 4,331,542, is inefficiency in structure of the underdrain unit, as to cost, bulk, number of parts and installation; and positioning of the air scouring or backflushing system at an elevation that is relatively high within the granular medium, such that lower portions of the granular medium are not well served by the scouring air, making backwashing less thorough or less efficient.
U.S. Pat. Nos. 5,545,358 and 5,526,620 are also pertinent to the subject matter of this invention, in describing tank covers in which structural elements of the covers serve as air or gas delivery or withdrawal conduits.